A number of motion sensors incorporating a ball within a housing have been developed over the years. For example, U.S. Pat. Nos. 3,742,478 and 4,196,429 describe motion sensors in which an electrically-conductive ball is confined for movement within a generally cylindrical container. On an inner surface of the container (either on the cylindrical side wall or on one or both of the end walls), two sets of elongated contacts are formed, the contacts of each set being interspaced or interlaced with those of the other set. The ball is able to bridge an adjacent pair of contacts to form an electrical connection between the two sets. As the device is moved, the ball rolls over the contacts, successively making and breaking connections between the two sets. This motion is detected by a circuit coupled to the contacts, and an alarm is then sounded.
The designs of U.S. Pat. Nos. 3,742,478 and 4,196,429 have a number of drawbacks. For example, a false alarm can arise from slight vibrations. A third set of contacts introduced to solve this problem in one embodiment of U.S. Pat. No. 3,742,478 results in a complicated electrode arrangement and makes the sensitivity of the device dependent upon the position of the ball prior to movement. Moreover, because the measuring circuit of those designs rely upon the ball thereof bridging adjacent contacts, the contacts have to be raised above the surface upon which the ball rolls. The movement of the ball and, accordingly, the sensitivity of the motion sensor is thus impeded. Furthermore, the elevated contacts result in a tendency of the ball to ride along between contacts rather than over the upper surfaces thereof, thereby preventing the alarm from sounding.
An improved motion sensor is described in U.S. Pat. No. 4,688,025, the disclosure of which is incorporated herein by reference. In one embodiment, that sensor comprises a ball confined within a cylindrical housing. The cylindrical housing comprises a side wall carrying a first set of electrical contacts and a second set of electrical contacts and an end wall carrying terminal means. The contacts of the side wall are preferably formed flush with the inner circumference of the side wall. During movement, the ball successively electrically connects the first set and the second of contacts with the terminal means. The sensor of U.S. Pat. No. 4,688,025 further includes a circuit for producing an alarm signal in response to detecting such successive connections.
Not only is the sensor of U.S. Pat. No. 4,688,025 well designed for the detection of motion, it also finds an important use as a detector for lack of motion. In that regard, the motion sensor of U.S. Pat. No. 4,688,025 is used in the FireFly.RTM. Personal Alert Safety Systems (PASS) available from Mine Safety Appliances Company of Pittsburgh, Pa. See, for example, Mine Safety Appliances Company Bulletin Nos. 0119-19, 0119-07, and 0119-06, as well as Mine Safety Appliances Company Data Sheet 01-00-22, the disclosures of which are incorporated herein by reference. The motion sensor of these PASS devices cause both audible and visual alarms to be activated if movement by a person wearing the PASS device ceases for a predetermined period of time. Likewise, U.S. Pat. No. 5,781,118, the disclosure of which is incorporated herein by reference, discloses a self-contained breathing apparatus having a PASS device wherein the motion sensor of U.S. Pat. No. 4,688,025 is integrated therewith.
PASS devices as described above are used, for example, as a safety device by firemen who may be overcome by smoke or toxic fumes while fighting a fire. Should such a fireman be overcome and cease movement, the PASS device provides an alarm to others so that a rescue can be effected. Recently, revisions to the National Fire Protection Association ("NFPA") Standard for PASS devices (NFPA 1982, 1998 Edition) were approved. As part of these revisions, the timing requirements for the detection of lack of motion are to be changed. In that regard, PASS devices will have to meet a tighter timing tolerance for the alarm.
It is thus very desirable to develop motion sensors that will meet and exceed the timing requirements of the NFPA Standard for PASS devices.